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4367. Improve optional<T&>::or_else

Section: 22.5.4.7 [optional.ref.monadic] Status: New Submitter: Hewill Kang Opened: 2025-09-07 Last modified: 2025-10-16

Priority: 4

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Discussion:

optional<T&>::or_else currently returns *val when it has a value, which calls the optional(U&&) constructor which in turn calls convert-ref-init-val which in turn calls addressof.

There's no reason to do that. It's much more efficient to call optional<T&>'s default copy constructor to just copy a pointer.

[2025-10-16; Reflector poll]

Set priority to 4 after reflector poll.

"NAD, no observable difference." "Still simpler to not have to reason about whether it matters, let's do it."

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.7 [optional.ref.monadic] as indicated:

   template<class F> constexpr optional or_else(F&& f) const;
   

   -7- Constraints: F models invocable.

   -8- Mandates: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true.

   -9- Effects: Equivalent to:

   if (has_value()) {
     return *thisval;
   } else {
     return std::forward<F>(f)();
   }